1. Field of the Invention
This invention relates to graft elastomers which are electrostrictive. It relates also to a process for producing strain in elastomers.
2. Description of the Related Art
Electromechanical coupling effects such as piezoelectricity and electrostriction have been utilized in a number of transducer and actuator technologies. Piezoelectric polymers and ceramics have been the typical candidates for actuation mechanisms. However, piezoelectric polymers such as poly(vinylidene fluoride) provide very low strains, while the ceramics are undesirably heavy and brittle, and provide low strains. To be sure, acceptable strains are now provided by electrostrictive polyurethanes and silicones. However, these polymeric materials have very low moduli, and consequently possess very little actuation authority.
Fluorine-containing graft copolymers have been known for some time. For example, U.S. Pat. Nos. 4,472,557 and 4,910,258 disclose the graft copolymerization of fluorine-containing elastomeric polymers with fluorine-containing crystalline polymers. Although it has been recognized that such graft copolymers possess desirable properties for certain applications, there has been no disclosure or suggestion in the art that such polymers could be transformed to provide a high strain energy density and therefore be suitable as actuation materials.
It is a primary object of the present invention, to provide what is not available in the art, viz., an electrostrictive polymeric material which provides the combination of high strain energy density, low mass, excellent conformability and processability, and mechanical and electrical toughness for durability. Such an electrostrictive material would be highly desirable as an actuation material for a wide variety of applications where electrically controlled mechanical motions are required.
It is also a primary object of the present invention to provide a process for producing an elastomer of high strain energy density, low mass, excellent conformability and processability, and mechanical and electrical toughness for durability.
These primary objects, and other attending benefits, are achieved respectively by the provision of two preferred embodiments of the present invention. In the first, an electrostrictive graft elastomer is provided having a backbone molecule which is a non-crystallizable, flexible macromolecular chain, and a grafted polymer forming polar graft moieties with backbone molecules, the polar graft moieties having been rotated by an applied electric field, advantageously into substantial polar alignment. The backbone molecule is advantageously a member selected from the group consisting of silicones, polyurethanes, polysulfides, nitrile rubbers, polybutenes, and fluorinated elastomers, e.g., a chlorotrifluoroethylene-vinylidene fluoride copolymer. The grafted polymer is a homopolymer or a copolymer, and the polar graft moieties are polar crystal phases and physical entanglement sites with backbone molecules. The grafted polymer is preferably a member selected from the group consisting of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) copolymers. Other suitable grafted polymers are poly(trifluoroethylene), vinylidene-trifluoroethylene copolymers, ferroelectric nylons (odd-numbered nylons), cyanopolymers (polyacrylonitriles, poly(vinylidene cyanide, vinylidene cyanide-based copolymers, poly(cyanoaryl ether)), polyureas, polythioureas, ferroelectric liquid crystal polymers and piezoelectric bipolymers. In a particularly preferred embodiment, the backbone molecule is a chlorotrifluoroethylene-vinylidene fluoride copolymer, and the grafted polymer is a poly(vinylidene fluoride) or a poly(vinylidene fluoride-trifluoroethylene) copolymer. The polar graft moieties, which are polar crystal phases and physical entanglement sites with backbone molecules, have been rotated by an applied electric field, advantageously into substantial polar alignment.
In the second preferred embodiment of the present invention, a process is presented which includes providing a graft elastomer having a backbone molecule which is a non-crystallizable, flexible macromolecular chain and a grafted polymer which forms polar graft moieties with backbone molecules. An electric field is applied to this graft elastomer to rotate the polar graft moieties, advantageously into substantial polar alignment, resulting in an elastomer of high strain energy density. In this process, the backbone molecule of the graft elastomer is advantageously a member selected from the group consisting of silicones, polyurethanes, polysulfides, nitrile rubbers, polybutenes, and fluorinated elastomers, such as chlorotrifluoroethylene-vinylidene fluoride copolymers. In this process, the grafted polymer is a homopolymer or a copolymer, preferably a member selected from the group consisting of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) copolymers, and the polar graft moieties are polar crystal phases and physical entanglement sites with the backbone molecules.
The electrostrictive graft elastomer of the present invention, which is the elastomer produced by the process of the present invention, is useful as an actuation material in applications where electrically controlled mechanical motions are required.